As general corrosion resistant materials there are stainless steels containing at least 11.00% of Cr, and in JIS G 4304 they are classified, depending upon their metallic structures, into five varieties, that is, austenitic, austeniteferritic, ferritic, martensitic and precipitation hardenable stainless steels. Among them ferritic stainless steels are relatively inexpensive and have enhanced workability and elongation, and therefore relatively large quantites of such steels are commercially used. Of the ferritic stainless steels, nine species of hot rolled sheets and ten species of hot rolled strips are standardized. Ten species of cold rolled sheets and strips are also standardized. Regarding the content of P of these standardized ferritic stainless steel sheets and strips, the standard prescribes 0.030% or less of P for two species of SUS 447 J1 and SUS XM 27 and 0.040% or less of P for other species.
A ferritic stainless steel has a crystalline structure of a body-centered cubic lattice which inherently leads to a reduced toughness and workability of the material. In addition, Cr contained in the material in an amount as high as at least 11.00% to provide the corrosion resistance also inherently acts to further reduce the toughness and workability of the material. Accordingly, regarding impurities which adversely affect the toughness and workability of the material, in particular P, the standard prescribes the strict provision of 0.040% or less of P.
In the production of thin products having a thickness of 4.0 mm or below, it has now been found according to the inventors' research that an adverse effect of P in excess of 0.040% upon the toughness of the products may be obviated by controlling amounts of Cr, C and sol. Al within appropriate ranges, respectively, and thus, it is possible to inexpensively supply corrosion resistant materials without sacrificing the corrosion resistance and mechanical properties of the products.
In producing stainless steels individual companies utilize their respective processes which basically involve melting of iron scraps, iron alloys and other materials in a electric furnace, refinining and adjustment of components in VOD, converter-VOD or AOD and casting of slubs and ingots. On the other hand, from a view point of productivity and saving energy, is also considered a process for producing stainless steels using an installation for the manufacture of ordinary steels wherein pig iron from a blast furnace is fed to a converter together with various subsidiary materials such as Fe-Cr alloys, in which converter refining and component adjustment are carried out. The pig iron normally contains substantial amounts of impurities such as P and S, and in particular 0.08 to 0.15% of P. In order that the product should contain a reduced level of P as low as 0.040% or below as in the standardized stainless steels, it is necessary to carry out a preliminary removal of P before the pig iron is fed to the converter or to carry out a special treatment for the removal of P in operating the converter, leading to a reduction of the productivity. If such treatments for removing P may be obviated, the productivity will be enhanced and the manufacturing costs will be reduced, rendering the process inexpensive. Accordingly, it can be understood that if the burden of controlling P prescribed in the standard of stainless steels may be lightened, it is possible to produce corrosion resistant alloys reduced in cost.